1. Field of the Invention
The invention relates to a cylinder lock, more particularly to improvements relating to a cylinder lock which includes pin tumblers that have cylindrical outer parts and concentric inner parts telescopically slidable within the outer parts.
2. Description of the Related Art
A first example of a conventional cylinder lock, similar to that disclosed in U.S. Pat. No. 1,095,500, is shown in FIGS. 1 and 2. A key (A) has a tumbler activating edge which is formed with a series of substantially V-shaped key bit grooves (only one key bit groove is shown). Each of the key bit grooves has a flat bottom (a) which engages the lower end of an inner part (b) of a respective pin tumbler. Each of the key bit grooves is defined by a pair of inclined sides (c) which engage the lower end of a concentric cylindrical outer part (d) of the respective pin tumbler. Note that the angle formed between the inclined sides (c) should be kept within a specified range. If the angle between the inclined sides (c) is relatively large, insertion or drawing of the key (A) from the keyway can be easily accomplished. However, the resulting difference (L) in the lengths of the inner part (b) and the outer part (d) of the pin tumbler is relatively small, thereby making it easier to pick the cylinder lock. If the angle between the inclined sides (c) is relatively small, difficulty in the insertion or drawing of the key (A) from the keyway is encountered. The resulting friction between the key (A) and the outer part (d) of the pin tumbler can reduce the effective life of the key (A).
A second example of a conventional cylinder lock, similar to those disclosed in U.S. Pat. Nos. 4,142,389 and 4,856,309, is shown in FIGS. 3 and 4. A key (B) has a planar face which is formed with a series of concentric recesses along a longitudinal axis thereof. The bottom (e) of an innermost recess of the key (B) engages the lower end of the inner part (f) of a respective pin tumbler. The bottom (g) of an outermost recess of the key (B) engages the lower end of the cylindrical outer part (h) of the respective pin tumbler. Note that the difference (L') in the depths of the innermost and outermost recesses should be within a specified range. Disengagement of the inner part (f) of the pin tumbler from the innermost recess of the key (B) is difficult to conduct if the difference (L') is relatively large. A relatively small difference (L'), however, will make it easier to pick the cylinder lock.
Note that the cylinder locks shown in FIGS. 1 to 4 can be easily picked if the difference in the lengths of the inner and outer parts of the pin tumbler is relatively small. Lock picking can be accomplished by inserting a vibrating picking tool into the keyway so as to activate the pin tumblers in sequence. A slight turning pressure is applied on the key plug as the cylinder lock is picked so that alignment of one end of the picked pin tumbler with the shear line can be maintained.
Another example of a conventional cylinder lock has been disclosed in U.S. Pat. No. 4,478,061. The cylinder lock includes two sets of tumblers. The tumblers of each set are aligned longitudinally and are arranged in a generally parallel co-extensive relationship. FIG. 5 illustrates how a key disclosed in U.S. Pat. No. 4,478,061 activates a pair of tumblers (M, N). The key can be U-shaped or H-shaped in cross-section and has a pair of teethed key blades which are adapted to activate the tumblers (M, N) independently.
Note that the tumblers of the cylinder lock disclosed in U.S. Pat. No. 4,478,061 are dissimilar from those installed in the first two examples. The cylinder lock is therefore more complicated and is more difficult to construct.